Industrial recirculating aquaculture system (RAS), as an emerging aquaculture technology driven by national facility fisheries policies, achieves intensification, high efficiency, and environmental sustainability in aquaculture through the integration of industrial engineering equipment and environmental control technologies. Its core advantages include: water recycling saving over 90% of water, independence from regional and seasonal constraints, precise regulation of key environmental factors such as water temperature and dissolved oxygen, significantly improving land productivity and feed conversion rates. It is recognized as a crucial direction for the sustainable development of aquaculture. Characterized by "high investment, high density, and high output," its widespread adoption is constrained by factors such as high initial investment (costs of facilities and equipment) and high technical barriers (seed acclimation and water quality management).
Mandarin Fish (Siniperca chuatsi), as a high-value freshwater aquaculture species, faces challenges in traditional farming such as frequent diseases, difficulty in water quality control, and unstable yields. Currently, technical reserves for industrial RAS of Mandarin Fish remain insufficient, especially lacking systematic practice in areas like farming process optimization, dedicated equipment design, and water purification processes. This research focuses on the efficient recycling and utilization of water resources, aiming to construct the process equipment system for land-based industrial aquaculture of Mandarin Fish. Through the optimization of low-disturbance waste discharge devices and the integration of equipment linkage technology, experimental research on key indicators such as water purification efficiency and bio-load capacity is conducted. The goal is to develop a replicable technical solution to support the high-quality development of the Mandarin Fish farming industry.
1. Industrial Recirculating Aquaculture Process Flow
The core of an industrial RAS is achieving dynamic water balance and recycling through a closed-loop process of "physical filtration - biological purification - disinfection and oxygenation". "Raising fish starts with raising water"; parameters such as water flow velocity, temperature, pH, ammonia nitrogen concentration, and dissolved oxygen level directly affect the growth environment of Mandarin Fish. This system design follows the principle of "small systems, multiple units". Its core logic is: faster flow rates can improve system processing efficiency, reduce the breakage of large particulate waste, and lower subsequent processing energy consumption; pollutant removal follows the sequence "solid → liquid → gas", solid waste treatment is graded by "large particle size → small particle size", and filtration and disinfection processes are sequentially connected.
As shown in Figure 1, the system flow is: drainage from the culture tank undergoes pretreatment to remove large particulate waste, enters coarse and fine filtration stages to remove fine suspended solids, then passes through a biofilter to degrade harmful substances like ammonia nitrogen, and finally, after disinfection and oxygenation, returns to the culture tank, achieving controlled water quality and water recycling throughout the process.
2. Design and Research on Mandarin Fish Aquaculture Facilities and Equipment
Traditional aquaculture facility design often relies on experience, easily leading to inefficient equipment and cost waste. As shown in Figure 2, this study, based on the principle of mass balance, constructs a model for the maximum biomass carrying capacity of Mandarin Fish. By calculating the maximum feeding rate, total waste, and ammonia nitrogen production, scientific equipment selection is achieved. Using a Mandarin Fish farming enterprise in Jiangxi as a case study, the focus was on optimizing the low-disturbance waste discharge device and the equipment linkage system. The workshop layout is shown in Figure 3. The layout of the land-based industrial RAS for Mandarin Fish is shown in Figure 4.
2.1 Culture Water Recirculation Parameter Design
The recirculation rate is key to efficient system operation and needs to be determined comprehensively based on Mandarin Fish stocking density, water volume, and water treatment capacity.
Water recirculation volume calculation formula: Q = V × N
Where: Q is the water recirculation volume (m³/h);
V is the culture water volume (m³);
N is the number of recirculations per day (times/d).
Culture Tank Design: Single tank diameter 6m, height 1.2m, cone bottom height 0.3m.
Calculated volume is π×3²×1.2 + 1/3×π×3²×0.3 ≈ 33.91 m³, actual culture water volume is about 30 m³. A single workshop contains 10 culture tanks, total water volume 300 m³.
Operating Parameters: Recirculation rate N is set at 3-5 times/d; makeup water circulation is 10% of the total water volume (to compensate for evaporation and discharge losses), adjusted in real-time through online monitoring.
2.2 Culture Tank and Waste Discharge Device Design
As shown in Figure 5, the culture tank is designed with the goals of "rapid waste discharge and uniform water distribution," using a circular tank body combined with a cone bottom structure. A "Fish Toilet" device is installed at the bottom to achieve low-disturbance waste discharge. The Fish Toilet was optimized as follows:
- Inlet/outlet pipe diameter standardized to 200mm to increase flow velocity.
- Cover plate adopts a rotating streamlined design to enhance the rotational flushing effect on bottom sediments and improve self-cleaning capability.
3. Solid Particulate Treatment Process Design and Research
Solid particles are treated by size classification using a three-stage process of "pretreatment - coarse filtration - fine filtration". Specific parameters are shown in Table 1.
3.1 Pretreatment Process
Utilizes a vertical flow settler linked with the side-drain and bottom-drain systems of the culture tank, using gravity separation to remove particles ≥100μm. The settler is directly connected to the culture tank to reduce pipeline transport losses and lessen the load on subsequent filtration stages.
3.2 Coarse Filtration Process
As shown in Figure 6, the coarse filtration process centers on a microscreen drum filter. Design principles include: locating the equipment close to the culture tanks to shorten pipeline length and reduce energy consumption.
Using a PLC control system to achieve automatic backwashing (4-6 times/d), coordinated with online water quality monitoring for real-time parameter adjustment.
Utilizing gravity flow design to reduce pump power consumption and lower operating costs.
3.3 Fine Filtration Process
As shown in Figure 7, the fine filtration process further purifies water quality through the synergistic action of the biofilter and disinfection equipment.
- Biofilter: Selects high-specific-surface-area media, hydraulic retention time 1-2h, maintains dissolved oxygen ≥5 mg/L, degrades ammonia nitrogen and nitrite.
- Disinfection Equipment: Ultraviolet sterilizer (dose 3-5 × 10⁴ μW·s/cm²) or ozone generator (concentration 0.1-0.3 mg/L, contact time 10-15 min) to kill pathogenic microorganisms.
- Oxygenation System: Pure oxygen oxygenator used in conjunction with aerators to ensure stable dissolved oxygen levels.
4. Pipeline Layout and Control System
4.1 Pipeline Layout Design
Pipelines are categorized by function into four types: water supply, recirculation, waste discharge, and makeup water. Design principles: Optimize layout centered around culture tanks, reduce elbows and pipeline length to minimize head loss; ensure balanced inflow and outflow to maintain stable water levels in culture tanks; waste discharge pipes have a slope (≥3%) to facilitate self-flow collection of waste.
4.2 Control System Design
The system adopts a closed-loop architecture of "Sensors - Controller - Actuators" as shown in Figure 8. Core functions include:
- Real-time water quality monitoring: Online data collection via dissolved oxygen, pH, and ammonia nitrogen sensors.
- Equipment linkage control: Automatic adjustment of microscreen backwashing, oxygenator power, and disinfection equipment runtime based on water quality parameters.
- Fault warning: Audible and visual alarms triggered by abnormal parameters, pushed to management terminals via Ethernet or wireless communication.
5. Equipment Performance Test Data Analysis
As shown in Figure 9, a six-month trial operation was conducted at a Mandarin Fish farming base in Jiangxi. The system experienced no water treatment abnormalities, and the monitoring and early warning system operated stably.
No water treatment abnormalities were found during application, the monitoring, early warning, and control system operated stably. Aeration in the culture tanks was used in combination with dissolved oxygen control during the farming process. The performance evaluation of the main equipment is shown in Table 2.
During the trial, the stocking density reached 50-60 fish/m³, survival rate ≥90%, growth rate increased by 20% compared to traditional farming, and the water recycling rate reached 92%, achieving energy saving and emission reduction goals.
6. Summary
The land-based industrial RAS for Mandarin Fish achieves the aquaculture goals of "water saving, high efficiency, and environmental protection" through the integration of engineering, facility-based, and digital-intelligent technologies. The innovations of this research lie in: optimizing equipment selection based on the biomass carrying capacity model to improve system matching; improving the low-disturbance waste discharge device to enhance waste removal efficiency; constructing an equipment linkage control system to achieve precise water quality regulation.
This system can be promoted and applied to other freshwater fish farming, providing a technical reference for the intensification transformation of aquaculture. Future work needs to further reduce equipment costs and optimize sensor performance to increase the technology penetration rate.